Printed circuit board assembly apparatus

ABSTRACT

Printed circuit board assembly apparatus comprises means for supporting a printed circuit board for assembly of components on one side of the board by inserting leads of the components into corresponding holes formed in the circuit board at predetermined locations, light projecting means for illuminating from the one side of the board the holes intended to receive the leads of a selected component and cutting means for cropping and crimping the leads of the component projecting from the other side of the board after assembly of the component on the board, the cutting means comprising a pair of cutters mounted on a rotatable carrier for linear movement in a pitch direction and each having a nip to be aligned with a corresponding hole in the board prior to the insertion of a corresponding component lead. Each of the cutters comprises an optical sensing arrangement having an optical axis disposed at a predetermined spacing from the nip of the cutter in the pitch direction for sensing light transmitted through a hole in the board with which the nip of the corresponding cutter is to be aligned, and control means are provided for first aligning the optical sensing arrangements with a pair of holes with which the nips of the cutters are to be aligned and then displacing the cutters by the predetermined spacing in the pitch direction to bring the nip of each cutter assembly into alignment with the respective hole in the board.

BACKGROUND OF THE INVENTION

THIS INVENTION relates to improvements in or relating to apparatus foruse in assembling electrical components on printed circuit boards.

A known form of apparatus used in the assembly of electronic componentson printed circuit boards comprises an inclined working frame which isconveniently arranged for a seated operative, and on which the printedcircuit board is secured. The printed circuit board has pre-drilledholes at predetermined locations for receiving the leads ofcorresponding components. An optical projector is mounted above theworking surface and is controlled by a micro-computer in accordance witha predetermined program to project light spots onto the printed circuitboard to indicate to an operator, in sequence, the positions at whichthe various components are to be assembled on the board. Means may alsobe provided to indicate the identity of the component to be assembled atthat location.

In use of such apparatus, the leads of the selected component areinserted by the operator into the holes indicated by the projector andthe projector program is then stepped to the next position. Thissequence of operations is repeated until all the required componentshave been assembled on the board.

It is then necessary to cut short the projecting leads on theundersurface of the board. This can be done by cropping and bending(clinching) each lead individually, thereby securing the assembledcomponents to the board. Alternatively, all of the assembled componentsto be cut may be held in position on the board by means of a resilientpad applied to the upper surface of the board, while all the leads arecropped in one sweep by a single wide cutting blade. In the latter case,because the leads are not clinched, it is then necessary to use someother means (commonly a shrink wrap foil) to secure the assembledcomponents while the board is soldered, commonly on a wave solderingmachine.

In practice, the individual lead cropping and clinching approach istime-consuming compared to the time required to assemble the componentson the board, while the single wide cutting blade approach requires someadditional means of securing the components prior to soldering andpresents significant manufacturing difficulties.

Because of the foregoing disadvantages, a variant of the opticallyassisted assembly apparatus has been developed in which the printedcircuit board (B) is movable with the working frame in mutuallyperpendicular X and Y directions under program control as illustrated inFIG. 2 of the accompanying drawings. Two cutters (C) are mountedsymmetrically under the printed circuit board on a carrier (K) which isvertically movable through a short (typically 5 mm) distance and whichis also rotatable about a fixed vertical reference axis (R). In use, thecircuit board is positioned so that the reference axis coincidessubstantially with the centre of the component to be inserted, that iscoincides with the mid-point between the corresponding printed circuitboard holes (H). The distance (pitch p) between the cutters can then beadjusted and the carrier rotated (θ) about the reference axis underprogram control so that nips of the cutters are aligned with the printedcircuit board holes. After alignment, the cutter assembly is raised tobring the cutters into contact with the underside of the printed circuitboard.

As the component leads ar inserted in the corresponding printed circuitboard holes, the leads also enter the nips of the cutters positionedbeneath the printed circuit board holes. On command, the cutters operateand crop the leads of the component to the required length and alsoclinch the cropped leads to secure the component to the board. Thisprocess is repeated in turn with each component. Thus, at the completionof the assembly operation, the board is immediately ready forflowsoldering without any possibility of components becoming detachedfrom the board.

In the known apparatus provided with such programmed cutters, in orderto accommodate very short component pitches, it is necessary to arrangethe cutters so that they can pass each other at the centre of rotationdefined by the reference axis. Because the minimum pitch required can beas small as 0.100 inches (2.54 mm), it can be seen that each cutter mustprotrude somewhat less than 0.050 inches (1.27 mm) beyond thecentre-line of the component lead involved. As illustrated in FIG. 3 ofthe accompanying drawings, such an arrangement implies that the line(cutter axis) joining the centres of the nips of the two cutters (C) isnot parallel with the line (hole axis) joining the centres of the twoprinted circuit board holes (H) and variable adjustment of the cutterassembly about the vertical reference axis (R) is required to compensatefor this and bring the cutters into alignment with the printed circuitboard holes. In effect, as shown on FIG. 3, the cutter axis is angularlyoffset from the hole axis by the variable angle tan⁻¹ 2.54/pitch wherethe pitch is measured in mms.

With these constraints, a difficulty arises as regards the size of thecutters. Thus, the cutters must be small enough to fit between the leadsof adjacent components which have already been assembled, so that thedesign of the printed circuit board is not constrained by thelimitations of the assembly machine and, at the same time, the cuttersmust be strong enough (therefore large enough) to withstand the cuttingloads encountered, which may be considerable when cutting maximumdiameter copper leads or leads made of harder materials.

It is found that, in order to provide sufficiently strong cutters and toreduce the interference between cutters and component leads alreadyassembled, it is necessary to minimise all the working clearances andthis implies that the cutters must be accurately aligned with thecorresponding holes by the programmed movements of the board and cuttersin order to permit entry of the component leads.

The accuracy of the programmed alignment is a function, inter alia, ofthe accuracy of the following factors in the apparatus:

a) The X and Y position including the precision of the drive means andthe straightness and perpendicularity of the guide means involved inpositioning the circuit board.

b) The distance between the cutters

c) The angle of the line joining the cutters with respect to X and Y.

With these factors alone, the required accuracy will only allow errorsin each element in the region of 0.025 mm, which is expensive toachieve.

In addition, there are sometimes significant errors in the drilling ofthe component holes in the printed circuit board. Such errors may not beconsistent between nominally identical boards and cannot therefore beeffectively compensated by adjustments to the programme.

Accumulation of these errors, possibly compounded by long term drift,sometimes makes it difficult for the operator to insert the leadsthrough the cutters and can involve time consuming individual adjustmentof the alignment of the cutters using manual joystick control.

In addition, the machine may require recalibration at intervals in orderto maintain sufficient accuracy.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a printed circuitboard assembly apparatus which enables the cutters of the apparatus tobe positioned reliably in alignment with respective component holes inthe circuit board.

Accordingly, in one aspect, the present invention provides printedcircuit board assembly apparatus comprising means for supporting aprinted circuit board for assembly of components on one side of theboard by inserting leads of the components into corresponding holesformed in the circuit board at predetermined locations, light projectingmeans for illuminating from the one side of the board the holes intendedto receive the leads of a selected component and cutting means forcropping and crimping a lead of the component projecting from the otherside of the board after assembly of the component on the board, thecutting means comprising a cutter having a nip to be aligned with acorresponding hole in the board prior to the insertion of acorresponding component lead, in which apparatus the cutter comprises anoptical sensing arrangement disposed at a predetermined spacing in apredetermined direction from the nip of the cutter, and control meansare provided for first aligning the optical sensing arrangement with ahole in the board with which the nip of the cutter is to be aligned andthen displacing the cutter in the said predetermined direction by thepredetermined spacing to bring the nip of the cutter into alignment withthe hole in the board.

The invention also provides printed circuit board assembly apparatuscomprising means for supporting a printed circuit board for assembly ofcomponents on one side of the board by inserting leads of the componentsinto corresponding holes formed in the circuit board at predeterminedlocations, light projecting means for illuminating from the one side ofthe board the holes intended to receive the leads of a selectedcomponent and cutting means for cropping and crimping the leads of thecomponent projecting from the other side of the board after assembly ofthe component on the board, the cutting means comprising a pair ofcutters mounted on a rotatable carrier for linear movement in a pitchdirection and each having a nip to be aligned with a corresponding holein the board prior to the insertion of a corresponding component lead,in which apparatus each of the cutters comprises an optical sensingarrangement disposed at a predetermined spacing from the nip of thecutter in the pitch direction for sensing light transmitted through ahole in the board with which the nip of the corresponding cutter is tobe aligned, and control means are provided for first aligning theoptical sensing arrangements with a pair of holes with which the nips ofthe cutters are to be aligned and then displacing the cutters by thepredetermined spacing in the pitch direction to bring the nip of eachcutter assembly into alignment with the respective hole in the board.

In the known printed circuit board assembly apparatus described above,the length of the component lead projecting from the lower surface ofthe printed circuit board after cutting is commonly controlled by meansof a rigid vertical locating pin fixedly mounted adjacent each cutter,so that the vertical distance between the underside of the printedcircuit board and the cutter is determined when the upper extremity ofthe locating pin is engaged by the lower surface of the printed circuitboard. In practice, after the circuit board has reached the nextinsertion position, the operator presses the board down to make contactwith the pins before initiating the cutting cycle. The height may bevaried by selecting and fitting suitable locating pins at the start ofan assembly job.

It will be appreciated that, when the board is pressed down to makecontact with the pin, there is a possibility that the pin may be driveninto forcible contact with the lead of a component already in position.Under some circumstances, this may result in a false height being setor, if the pin is displaced laterally by the resulting force, may causefracture of the necessarily slender pin.

It is another object of the present invention to provide a printedcircuit board assembly apparatus in which the length of the trimmedcomponent leads may be reliably set.

Accordingly, in another aspect of the invention, there is providedprinted circuit board assembly apparatus comprising means for supportinga printed circuit board for assembly of components on one side of theboard by inserting leads of the components into corresponding holesformed in the circuit board at predetermined locations, cutting meansfor cropping and crimping the leads of a component projecting from theother side of the board after assembly of the component on the board andmeans for positioning the board in a cropping position relative to thecutting means prior to actuation of the cutting means, the cutting meanscomprising locating pin means projecting from the cutting means forengaging the said other side of the board in the cropping position todetermine the length of the cropped leads, in which apparatus thelocating pin means comprises a locating pin pivotally mounted on thecutting means and resiliently biased into a normal position from whichit is pivotally displaceable against the resilient bias by an obstacleon the said other surface of the board so as to engage the surface ofthe board.

In use of the known printed circuit board assembly apparatus describedabove, it is important that the severed ends of the component leads areremoved from the cutting area without any possibility of jamming in themechanism. This is achieved using a downwardly tapering scrap chute, thewider upper end of which is located directly beneath the cutters and thenarrow lower end conducting the severed ends of the leads out of thebottom of the mechanism. This chute merely serves the purpose ofremoving the severed ends of the leads and a separate drive mechanism isprovided for actuating the cutters.

A further object of the present invention is to simplify this actuationof the cutters.

Accordingly, in a third aspect, the invention provides printed circuitboard assembly apparatus comprising means for supporting a printedcircuit board for assembly of components on one side of the board byinserting leads of the components into corresponding holes formed in thecircuit board at predetermined locations, cutting means for cropping andcrimping the leads of a component projecting from the other side of theboard after assembly of the component on the board and chute means forcollecting the ends of the leads severed by the cutting means, in whichapparatus the chute means is movable relative to the cutter means toactuate the cutting means to crop and crimp the component leads.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood, anembodiment thereof will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the operating principle of aprinted circuit board assembly apparatus embodying the presentinvention;

FIG. 2 is a diagram illustrating the programmed movements necessary inorder to align two component lead cutters carried on a rotatable carrierwith respective holes in a printed circuit board;

FIG. 3 illustrates geometrical the arrangement of two component leadcutters which are able to pass at the centre of rotation of the carrier;

FIG. 4a-4d shows an optical detector employed in the apparatus embodyingthe invention under four different conditions of illumination by a lightspot;

FIG. 5 is a partially sectioned side elevation of the main elements of aprinted circuit board assembly apparatus embodying the presentinvention;

FIG. 6 is a cross-sectional view taken through the apparatus of FIG. 5on the line VI--VI of FIG. 5;

FIG. 7 is a plan view of the apparatus;

FIG. 8 is a perspective of a scrap chute of the apparatus;

FIG. 9 illustrates the normal condition of a locating pin employed inthe apparatus embodying the invention;and

FIG. 10 shows the locating pin in a pivotally displaced position as aresult of encountering an obstruction on the lower surface of a printedcircuit board.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to FIG. 1, a printed circuit board 1 is supported by aworking frame 16 in a working position for the assembly of electroniccomponents on the upper surface of the board by insertion of the leadsof each component into a pair of corresponding pre-drilled holes 2 and 3in the circuit board, so that the component leads project from a lowersurface of the board.

An optical projector 4 is arranged to project, in accordance with apredetermined program, a spot of light onto each of the pair of holes 2and 3 corresponding to the component to be assembled on the board.

Cutting and clinching means 5 is disposed beneath the circuit board 1and comprises a pair of cutters 6 each for positioning beneath arespective one of the holes 2 and 3 for cropping an end portion of acomponent lead projecting through the respective hole and clinching theremainder of the lead to secure the component to the board during asubsequent soldering operation. The cutters 6 are mounted on a rotatablecarrier (not shown) and are linearly movable towards and away from oneanother in a pitch direction 7. The cutters are offset from one anothertransversely of the pitch direction so that the cutters are movable pastone another in the pitch direction.

An optical position sensing system 8 is associated with, and movablewith, each of the cutters 6 and serves to detect the spot of lightprojected through the respective component hole 2, 3 in the board 1.Each optical system 8 has an inlet aperture 9 defining a verticaloptical axis located at a predetermined offset d in the pitch directionfrom a nip 10 of the cutter. Reflecting means in the form of a mirror orprism 11 deflects the light beam admitted by inlet aperture 9 through90°, to a lens 12 for focussing the deflected beam onto a light detector13 which delivers output control signals to a microcomputer 17.

The light detector 13 is a monolithic quadrant silicon photodiode, suchas the device made and sold by Integrated Photomatrix Limited as deviceIPL 10130. As illustrated in FIG. 4, such a device has a circular lightsensitive area divided into four quadrants constituting a pair ofopposed X quadrants 14A, 14B and a pair of opposed Y quadrants 14C and14D. The quadrants are electrically connected together to provide afirst X error output signal which is significant of any differencebetween the illumination of quadrants 14A and 14B and a second Y erroroutput signal which is significant of any difference in the illuminationof quadrants 14C and 14D.

FIG. 4a shows a light spot 15 imaged centrally on the detector 13,indicating alignment of the optical axis of the position sensing system8 with the corresponding hole in the printed circuit board. In thiscondition, the detector produces no output signal. FIG. 4b shows theposition of the light spot 15 on the detector when there is an X errorin the alignment of the position sensing system 8 and the correspondinghole, while 4c shows a position of the light spot when there is a Yerror in the alignment and FIG. 4d shows a position of the light spotwhen there are both X and Y errors in the alignment. Any output errorsignals from the detector 13 serve as control signals which are fed tothe micro-computer 17 and are used to make automatic adjustments to theposition of the circuit board, the rotatable carrier and the cutters toachieve the zero error position of

FIG. 4a, at which time both of the optical position sensing systems aretruly aligned with the component holes, regardless of minor machine orprinted circuit board errors or drift. Finally, the cutters themselvesare accurately brought into alignment with the holes 2 and 3 by simplydisplacing the cutters by the fixed distance d in the pitch direction sothat they replace the optical position sensing system. The componentleads can then be inserted into the holes 2, 3 and into the nips 10 ofthe underlying cutters 6 and the projecting leads cropped and clinchedby closing operation of the cutters 6.

FIGS. 5 to 10 illustrate in more detail the cutter assembly of anapparatus embodying the invention. Referring to these Figures, a cuttercarrying disc 21 is attached to a bottom plate 23 by side plates 22. Thebottom plate 23 is supported by a ball journal 24 on a frame 25 whichalso supports the disc 21 in a roller bearing (not shown), so that thecarrier disc 21 is mounted for rotation about a vertical reference axis(R). Elevating means (not shown) are provided for raising the frame 25by a short distance, typically 5 mm, towards a printed circuit board 20supported above the disc 21 in a working position.

The side plates 22 carry guide rods 26 and lead screws 27 which supportand position two identical cutters 28 symmetrically disposed above theaxis of rotation. Each lead screw 27 is rotatable by an electrical motordrive (not shown) to displace the associated cutter 28 linearly in apitch direction in accordance with a control signal applied to thedrive. Each cutter 28 has a fixed jaw 29 and a movable jaw 30 definingtherebetween a nip 31, the movable jaw 30 being mounted on an operatinglever 32 pivotally mounted on the cutter 28 and actuated by a roller 33.

Each of the rollers 32 runs on a respective rail 34 which is attached tothe upper extremity of a scrap chute 35 which has inclined inner walls36 and lifting spigots 37. The chute 35 is constrained to move in avertical direction only by suitable means (not shown).

Each cutter 28 comprises, as shown more particularly in FIG. 6, anoptical position sensing arrangement 40 comprising a slot 41 defining aninlet aperture with an optical axis located from the centre of theassociated cutter nip. A mirror 42 is disposed at a predeterminedspacing d in the pitch direction at the bottom of the slot 41 anddirects the light emerging from the hole in the printed circuit board toa lens 43 which focusses the image of the illustrated hole onto anoptical detector 44 of the kind already described with reference to FIG.4. Misalignment between the programmed position to which the printedcircuit board hole is moved and the optical axis of the position sensingarrangement causes an identical displacement of the spot of illuminationfalling on the detector 44, resulting in an output electrical errorsignal from the sensor which is fed to the micro-computer 16 which,depending upon the error signal generated, acts to adjust the X, Yposition of the board, the rotational position of the disc 21 and thelinear position of the cutters 28 to align the optical axis of eachposition sensing system with the centre of the respective hole in theprinted circuit board. Once this has been achieved, each cutter 28 issimply and accurately positioned beneath the hole in the printed circuitboard by automatically linearly displacing the cutter block by thepredetermined spacing d under the control of the computer.

The cutter assembly is then elevated and the leads of a component may beinserted into the holes in the circuit board 20 so that the leadsproject beneath the board into the nips of the aligned cutters 28. Theboard 20 is then pressed down into engagement with locating pins 18which are mounted on the cutters 28 and define a predetermined spacingbetween the bottom of the board and the cutter jaws, so as to determinethe length of the component leads after cropping by the cutter assembly.The cutters 28 are then operated to carry out the cropping and clinchingaction by urging the chute 35 upwardly by means of a drive mechanism(not shown) acting on the spigots 37. This upward motion of the chute 35causes the levers 32 to pivot upwardly, regardless of the position ofthe cutters with respect to the axis of rotation, and accordingly closesthe movable jaw 30 of the cutter device against the fixed jaw 29 to cropand clinch the component lead. Means (not shown) are also provided toensure that downward movement of the chute 35 results in re-opening ofthe cutter jaws 29, 30.

FIGS. 9 and 10 illustrate a preferred form of the locating pin 18 fordetermining the length of the cropped leads. As shown in these Figures,the locating pin 18 comprises a cylindrical body 50 formed with an uppertapered end portion 51. The body 50 of the pin is housed in acorresponding blind bore 52 of larger diameter formed in the cutter. Alower end 53 of the pin body 50 is rounded and fits closely in acomplementary depression in the base of the bore 52, so that limitedpivoting movement of the pin 18 about the end 52 within the bore 52 ispossible. The open end of the bore 53 is further enlarged to form anannular seat 54 fitted with a resilient 0-ring 55 which resilientlyengages an upper end portion of the cylindrical pin body in a normalupright position of the pin 18. The upper end portion 51 of the pinprojects from the bore and has a free upper end which is intended to beengaged by the lower surface of the printed circuit board 20.

Such a locating pin 18 enables accurate location of the printed circuitboard 20 with respect to the cutters 28, even when the pin 18 encountersa lead 60 of an already installed component. In this case, the pin 18 issimply pivotally displaced to one side of the component lead 60 andengages the board surface next to the lead 60 as shown in FIG. 10.

In another embodiment of the invention (not illustrated), the opticalposition sensing system 8 is replaced by a light guide and a simpleunitary light detector delivering a single output signal indicative ofwhether or not the detector is illuminated. The light guide conductslight falling on its input end to the light detector which is closelycoupled to an output end of the guide.

In operation of this alternative position sensing system, once a printedcircuit board hole has been positioned at its pre-programmed locationabove the cutting means, the circuit board is automatically subjected toa small checking displaecment in the X direction, such that the lightspot from the hole is swept over the inlet end of the light guide,causing the detector to be subjected to two transitions between lightand dark conditions. The output signal of the detector is monitoredduring this checking displacement of the board and the positions ofthese two transitions are recorded. The desired corrected X co-ordinateof the circuit board hole is then determined as the mid-point betweenthe positions of the two transitions and the board is positionedaccordingly. A corrected Y position co-ordinate is then similarlydetermined by automatically subjecting the board to a checkingdisplacement in the Y-direction in order finally to position the holeaccurately with respect to the optical position sensing system.

We claim:
 1. Printed circuit board assembly apparatus comprising: means for supporting a printed circuit board for assembly of components on one side of the board by inserting leads of the components into corresponding holes formed in the circuit board at predetermined locations; light projecting means for illuminating from the one side of the board the holes intended to receive the leads of a selected component: and cutting means for cropping and crimping a lead of the component projecting from the other side of the board after assembly of the component on the board, the cutting means comprising a cutter having a nip to be aligned with a corresponding hole in the board prior to the insertion of a corresponding component lead: the cutter comprising an optical sensing arrangement having an optical axis disposed at a predetermined spacing in a predetermined direction from the nip of the cutter; control means being provided for first aligning the optical sensing arrangement with a hole in the board with which the nip of the cutter is to be aligned and then displacing the cutter in the said predetermined direction by the predetermined spacing to bring the nip of the cutter into alignment with the hole in the board.
 2. Printed circuit board assembly apparatus comprising: means for supporting a printed circuit board for assembly of components on one side of the board by inserting leads of the components into corresponding holes formed in the circuit board at predetermined locations; light projecting means for illuminating from the one side of the board the holes intended to receive the leads of a selected component; and cutting means for cropping and crimping the leads of the component projecting from the other side of the board after assembly of the component on the board, the cutting means comprising a pair of cutters (28) mounted on a rotatable carrier for linear movement in a pitch direction and each having a nip to be aligned with a corresponding hole in the board prior to the insertion of a corresponding component lead; each of the cutters comprising an optical sensing arrangement having an optical axis disposed at a predetermined spacing from the nip of the cutter in the pitch direction for sensing light transmitted through a hole in the board with which the nip of the corresponding cutter is to be aligned; control means are provided for first aligning the optical sensing arrangements with a pair of holes with which the nips of the cutters are to be aligned and then displacing the cutters by the predetermined spacing in the pitch direction to bring the nip of each cutter assembly into alignment with the respective hole in the board.
 3. Apparatus according to claim 2, in which the cutting means comprises locating pin means projecting from the cutting means for engaging the said other side of the board to space the cutting means from the board and determine the length of the cropped leads, the locating pin means comprising a locating pin pivotally mounted on the cutting means and resiliently biased into a normal position from which it is pivotally displaceable against the resilient bias by an obstacle on the said other surface of the board so as to engage the surface of the board.
 4. Apparatus according to any one of claim 2, comprising chute means for collecting the ends of the leads severed by the cutting means, in which apparatus the chute means is movable relative to the cutting means to actuate the cutting means to crop and crimp the component leads. 